Coupling Molecular dynamics to a continuum in LibMultiScale: how to address thermal and dislocation issues
Guillaume Anciaux (EPFL), Srinivasa Ramisetti (EPFL), Till Junge (EPFL), Jean-François Molinari (EPFL)
From Atomistics to Reality: Spanning Scales in Simulations and Experiments Symposium A
Tue 9:00 - 10:30
CIT 165
Many phenomena in crystalline metals such as friction and ductile fracture are poorly understood. Molecular Dynamics (MD) is widely used to model condensed matter. Nevertheless, capturing the multiscale nature of important processes necessitates sizes which are often out of reach of modern computers. Thus, multiscale approaches have emerged to reduce the computational cost of MD by using a coarser continuum model (CM). The difference between MD and CM leads to problems that LSMS co-workers addressed within our coupling framework: LibMultiScale. In this presentation, finite temperatures and dislocation passing will be addressed. In the case of dynamic simulations, the appearance of spurious wave reflections across coupling interfaces is a common artefact. A mode-based damping is necessary to prevent reflections of the waves not supported by the CM. This damping-coupling approach leads to energy leaks evidenced by processes generating high heat rates. This is why most finite temperature implementations use classical thermostats and can only model thermal equilibrium. For heat flows, a local energy balance must be achieved between the energy transmitted to the CM, the dissipated energy, and the energy that "missing atoms" must return. We use the Generalized Langevin Equation where the restitution balance a selective damping based on spatial filter functions and allow a precise control of the dissipated modes. Dislocations which nucleate due to localized stresses and move over large distances are another major issue. We currently develop an extension to 3D of the Coupled Atomistics and Discrete Dislocations method. Dislocations approaching the coupling interface are detected from the MD while pad atoms in the CM serve as boundary conditions. In 3D, dislocation lines can span from MD and CM regions which impact pad atoms. We employed "templated" core structures to provide boundary conditions to a moving straight edge dislocation crossing the coupling region.